Band-pass transformer



Nov. 2, 1948. GAINER BAND-PASS TRANSFORMER 2 Sheets-Sheet 1 Filed Aug.11, 1945 FIG. 3

Nov. 2, 1948. J. B. GASNER BAND-PASS TRANSFORMER 2 Sheets-Sheet. 2

Filed Aug. 11, 1945 FIG.4

FIG.5

ammo/whom JOSEPH B. GAINER Patented Nov. 2, 1943 UNITED STATES PATENTOFFICE (Granted under the act of March 3, 1883, as amended April 30,1928; 370 0. G. 757) 10 Claims.

My invention relates to transformers and particularly to transformersutilized in radio-frequency amplifier or oscillator circuits.

In accordance with my invention, the primary and secondary windings of atransformer are mounted in a fixed relative position affording greaterthan optimum or critical coupling and a tertiary windin inductivelyrelated to both the primary and secondary windings is adjustable toreduce their effective mutual inductance to an extent affording thedesired frequency-response characteristic, of an interstage transformerfor example, or to afford the desired amount of a feedback in anoscillator, detector or amplifier system.

My invention further resides in features of construction and arrangementhereinafter described and claimed.

For an understanding of my invention and illustration of embodimentsthereof, reference is made to the accompanying drawings, in which:

Fig. l, in perspective, shows an interstage transformer.

Fig. 2 is an explanatory figure mentioned in discussion of Figs. 1 and3.

Fig, 3 schematically shows a portion of an amplifier system using thetransformer of Fig. 1.

Fig. 4, in perspective, shows another type transformer.

Fig. schematically shows an oscillator system using the transformer ofFig. 4.

Referring to Fig. l, the primary winding P and secondary winding S of aninterstage transformer T are wound respectively on the tubular forms land 2 suitably fastened to the base member 3. The coil and base assemblyis enclosed by a conductive shield 4 provided with lugs, not shown,which pass through notches 5 in the base 3 and through the receiverchassis, or equivalent. not shown. The terminals 6 to 9 fastened to andextending through base 3 provide for external'connection of thetransformer windings to other circuit elements, for example, Fig. 3, tothe output and input electrodes respectively of amplifiers tubes V1, V2for implicity only shown as triodes. The inductances of the windings Pand S may be and usually are more or less exactly equal.

It is usually desired that the frequency-response curve of such aninterstage transformer as tuned by condensers C1, C2 and/or by the cores!2, 13 shall have the general shape of curve ill, Fig. 2, characterizedby a relatively flat top and steep sides to provide for substantiallyuniform amplification of all frequencies throughout a band offrequencies ii to f2 and with great attenuation for all frequencieslower and higher than if and f2 respectively.

Because of the interdependent variables involved, it is difiicult todesign and manufacture radio-frequency transformers which have thedesired bandpass characteristics both as to width of band and limitingfrequencies: moreover when the desired band-width is but a smallfraction of the mean frequency in, the required coupling is so small thecorresponding relatively great physical separation of the windingsenforces, when the usual practice is followed, large overall dimensionsof the transformer.

In accordance with my invention however, the windings P and S arepurposely positioned close to one another affording a percent couplingwhich is substantially in excess of optimum or critical and resulting ina band-pass characteristic generally as exemplified by curve ll, 2. Toreduce the mutual inductance of transformer '1, there is provided atertiary winding W comprising one or more turns coupled to and slidablealong the primary and secondary windings P and S.

By adjustment of winding W along the coils P and S, thefrequency-response characteristic may be varied from one havinggenerally the shape of curve H to one having generally the shape of thecurve l6. Winding VJ may be secured in a properly adjusted position by asuitable coil cement, tape or the like. It is understood that thephysical position of winding W may be altered to produce either anadditive or substractive effect on the coupling between coils P and Sas, by reason of the closeness of coupling between the primary cell Pand one part of the winding W and between the secondary coil S and theother part of the winding W, the winding W can be adjusted to haveconsiderable effect on the coupling between coils P and S so that it maybe adjusted to the point Where it not only reduces the normallyover-coupled condition between coils P and S to zero but actuallyproduces a reversal of phase in the output of coil S. The two parts ofwinding W may be cross-connected, as shown, so that the part on form 2opposes the secondary coil S or the part of winding N on form 2 may bewound in a direction to aid the secondary coil S when it is desired toincrease the mutual coupling between coils P and S. The adjustment of Walso can be used to compensate for stray coupling of coil leads orcomponents such as condensers or resistors within the shield 4.

Variation of the mutual inductance of two circuits individually tuned tosame frequency, substantially equally and simultaneously shifts theupper and lower resonant frequencies is and f4 toward or away from themedian frequency. Consequently although the aforesaid adjustment ofwinding W may afford the desired band-pass requirements and shape ofcurve ill, the position of the frequency band passed may not exactlycoincide in the frequency spectrum with the desired band because ofmanufacturing Variations or tolerances in self inductance of windings Pand S.

Preferably therefore, the self-inductance of 3 each of windings P and Smay be individually adjusted by cores l2 and I3 which may be of magneticmaterial, such as powdered iron, or of nonmagnetic material such ascopper or brass, or in the who-1e or part of dielectric, the choice ofIna-.- terial depending at least in part on the position in thefrequency spectrum at which the trans--' former is to operate. and 13,the upper and lower frequencies is and f4 may be individually shifted sothat in combination with winding W, the desired band width and thedesired upper and lower frequencies of the band may be accuratelydetermined without need for precise manufacturing tolerances andconstruction,

As shown in Fig. 1, the cores may be mounted within-and rather closelyfit the tubes l and 2 for adjustment by the rods l4, l5 which areengaged by threaded inserts, I5, l'l, Fig. 1 secured to the upper endsof forms I and 2. This transformer has the desirable feature that allfreely variable tuning adjustments are from the same side of thetransformer-and usually outside of the chassis, the winding W beingfixedly secured in position by cement, tape, or the like when finallyadjusted before the shield 4 is placed in position.

Preferably the rods l4, l5 of Fig. 1 and metallic parts electricallyconnected thereto are grounded through spring material 58 and the shieldcan 4 to avoid effects of external coupling and capacity duringalignment.

In amplifiers used in television receivers for example where theband-width is large, for example 4 to 6 megacycles as contrasted with amaximum of 10 or kilocycles for broadcast recep-- tion, dampingresistors, such as resistances l9 and 26, may be used.

By way of example, the dimensions of the transformer T for use atfrequencies between 50-56 megacycles are as follows:

Overall height inches 1 /8- Overall diameter "inches" 1 Coil formsdiameter inch Coil forms length inches 1 Coil form turns (#26 enameled-wire) 11 Center to center of coils PS inch Tertiary winding turns 2'Condenser C1, C2 mmf 7-10 Resistor pri ohms 1800 Resistor sec "ohms"2700 From the foregoing dimensions, it is apparent that the coil forms Iand 2 are spaced from each other about one-eighth of an inch. Thus, ascan be seen from Fig. l, the two partsof winding W have very little.mutual inductance therebetween because the turns onform Iarepractically in the same plane as those on form 2 and the crossedconnections between them are very shortand cross each othersubstantially at right angles. The latter feature is of considerableimportance in eliminating mutual inductance between the parts of coil astherecan be little inductance between very short leads and there is noinductance. between leads whichcross at right angles in that there canbe no interaction between the magnetic fields surrounding such leads.

In absence of an oscilloscope and a sweepfrequency generator suited forwide-band alignment, a signal generator and output meter constitute theonly test equipment needed for attainment of proper adjustment of thetertiary winding and cores.

The transformer exemplified in Fig. 4 is for use in an-oscillatorcircuit such for example as shfiwn By adjustmentof coresl2..

in Fig. 5. The construction of transformer T-1, is generally similar totransformer T of Fig. 1 and accordingly need only briefly be described.

The primary and secondary =coils P1 and S1 are Woundrespectively onforms l9 and 20 disposed side by side and extending from base 2|.Ordinarily the primary turns are fewer in number thanthe secondary butin any event the mutual coupling of these windings is purposely greaterthan" that required to sustain oscillations.

The mutual coupling may effectively be reduced in control of thefeed-back voltage introduced by coil P1 into the tuned grid circuit Cs.S1, Fig. 5, by adjustment of the tertiary coil W1 which for ease ofcontrol is wound on tubular forms 26, 2'! slidable respectively alongforms I9, 20 and connected by the insulating bridging member 28. Theknob 29 and rod 30 connected to member 28 permit adjustment externallyof the transformer shield can 3| of the position of the feed-backcontrol link W1. Again the phase of the tertiary winding may be to aidor oppose the mutual coupling.

A similar transformer may be used in a regenerative or degenerativeamplifier or detector stage in which the feed-back voltage may bedetermined or adjusted by a tertiary winding W1. Such transformer mayfor example be used to advantage in a regenerative detector circuitutilizing a screen grid tube, Fig. 5. Ordinarily in such circuit,feed-back is controlled by varyingthe screen grid voltage to maintainthe operating point (i. e. below the threshold of oscillation). Thechief difiiculty or objection is that the voltage supplied to the screenmay not correspond with the operating point of the tubes characteristiccurve affording the desired or maximum sensitivity. With the presentarrangement the screen voltage may be set independently at a point formaximum. sensitivity and the feed-back independently controlled byadjustment of link Wi. In addition, this construction in common withthat of the interstage transformer allows for a reduction on thephysical size. of the unit andv elimination of need for close tolerancesin manufacture.

Typical values for an oscillator or a regenerative detector embodied inFig. 5 operating between 30-50 megacycles with tetrode V1 follow:

P1 6 turns (#22 wire) S1 15 turns (#22 wire) Forms diameter..

Spacing between coil centers l' 2 turns (#22 Wire on "forms) 250micro-micro-farads (fixed) CZ 50 micro-microfarads (variable) R1 33,000ohms (grid resistor) C3; .1"micro-farad (fixed) L1 High impedance choke50.

henriesi compactness the primary and secondary windings are themselvesovercoupled.

The invention described herein may be manufactured and used by or forthe Government of the United States of America for governmental purposeswithout the payment of any royalties thereon or therefor.

What I claim is:

1. A structure comprising a radio frequency transformer includingprimary and secondary windings inductively coupled and disposed in closeside by side relation with their axes substantially parallel, a two-loopclosed circuit tertiary winding slidably mounted on said structurehaving at least one turn in each of its loops and one of its loopssubstantially coaxial with said primary winding and the other of itsloops substantially coaxial with said secondary winding, said tertiarywinding bein adjustable as a unit towards and away from adjacent ends ofsaid primary and secondary windings to effect thereby a variation in themutual coupling between the primar and secondary windings.

2. The radio frequency transformer of claim 1 in which the tertiarywinding is arranged to produce a flux in opposition to the flux linkagebetween said primary and secondary windings.

3. The radio frequency transformer of claim 1 in which the tertiarywinding is arranged to produce a flux aiding the flux linkage betweensaid primary and secondary windings.

4. The radio frequency transformer of claim 1 includin means fordetermining the location in the frequency spectrum of the band offrequencies passed by said transformer comprising cores adjustable tovary the self-inductance of said primary and secondary windings.

5. A structure comprising a radio frqeuency transformer includingprimary and secondary windings inductively coupled and disposed in closeside b side relation with their axes substantially parallel, means fordetermining the location of the frequency band passed by saidtransformer comprising means to adjust the self inductance of at leastone of said windings, a twoloop closed circuit tertiary winding slidablymounted on said structure having at least one turn in each of its loopsand one of its loops substantially coaxial with said primary winding andthe other of its loops substantially coaxial with said secondarywinding, said tertiary winding being adjustable as a unit towards andaway from adjacent ends of said primary and secondary windings to effectthereby a variation in the mutual coupling between the primary andsecondary windings.

6. The radio frequency transformer of claim 5 in which the tertiarywindin is arranged to produce a flux in opposition to the flux linkagebetween said primary and secondar windings.

7. A. radio frequency transformer comprising first and second windingforms disposed in close side by side relation with their axessubstantially parallel, a primary winding disposed on said first form, asecondary winding disposed on said second form substantially injuxtaposition to said first winding and inductively coupled thereto, atwo-loop closed circuit tertiary winding having at least one turn ineach of its loops and one of its loops disposed on said first formsubstantially coaxial with said primary winding and the other of itsloops disposed on said second form substantially coaxial with saidsecondary winding, said tertiary winding being adjustable as a unitalong said forms towards and away from adjacent ends of said primary andsecondary windings to effect thereby a variation in the mutual couplingbetween the primar and secondary windings.

8. The radio frequency transformer of claim 7 in which the tertiarywinding is arranged to produce a flux in opposition to the flux linkagebetween said primary and secondary windings.

9. The radio frequency transformer of claim 7 including means fordetermining the location in the frequency spectrum of the band offrequencies passed by said transformer comprising cores adjustablewithin said winding forms to vary the self-inductance of said primaryand secondary windings.

10. The radio frequency transformer of claim 9 in which the tertiarywinding is arranged to produce a flux in opposition to the flux linkagebetween said primary and secondary windings.

JOS. B. GAINER.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 2,051,012 Schaper Aug. 11, 19362,106,226 Schaper Jan. 25, 1938 2,144,214 Below Jan. 1'7, 1939 2,206,041Moore et a1. July 2, 1940 2,216,874 Case Oct. 8, 1940 2,258,147 RobertsOct. 7, 1941 2,383,475 Dodington Aug. 28, 1945

